High speed printers of the type in which this invention is most useful generally comprise a plurality of electromagnetically operated print hammers which strike individual type on a revolving type carrier such as a flexible belt or a rotating drum. A solenoid actuating coil respectively associated with each hammer operates the hammer when the coil is energized by an electronic drive circuit. The actuating coils and their drive circuits are connected via a circuit breaker to a current source for that purpose. The print control, which may include an electronic data processor device such as a microprocessor, selectively operates the print hammers in a random sequence to record a line of characters received from a data processing machine onto a record medium which is then incremented on one or more line spaces at the completion of printing. In printers using a flexible type belt, the type pitch and the hammer pitch differ so that the type aligns with the hammers in subscan sequences. The control system includes scan circuitry which options the drive circuits in the same subscan sequences. The same scan circuitry reads binary coded print data from a buffer for comparison with binary coded signals indicating the characters on the type belt. When comparisons occur, hammer fire signals are generated which turn on the optioned drive circuits to energize the coils with current from the power source for selected hammer operation. The drive circuits, when turned on, generate a feedback signal also called an echo signal. Among other things the echo signals are used during printing by checking circuitry for individually checking the hammer operation.
Periodically during the life of an impact printer, a fault condition associated with a specific print position may occur when the machine is operating in the non-print mode. Such a condition can occur during start up and prior to a print command or between print commands from the processor. Previous printer control systems provide means for sensing an electrical fault when not printing and immediately disconnecting power to protect the hammer units. The location and identification of the fault is done sometime later when the location of the fault becomes more difficult and, in cases where the fault is intermittent, impossible. The difficulty of locating the fault is even greater in those printers where the operating circuits and controls are part of an integrated circuit package having a large number of operating circuits and electrical connections on a single substrate or printed circuit device.